I fixed the transducer next to the sea water intake on the port side . It is some 1.2 m (3.9 feet) from the bottom of the keel. There may be some various depth of the keel . To play safe , you can allow the draft ( 5.5 feet)of the boat as the distance from the bottom of the keel to the transducer. You get some haft meter ( 1.6 feet) safe margin. Set the alarm 3 feet or more below the keel. It depends on the water depth of your mooring location. I have only one foot clearance during the extreme low tide in a year .